The strength increase (high tension) of a steel sheet used is one of the most effective methods to achieve both weight saving and a collision safety for a vehicle body. Recently, a regulation regarding the collision safety represented by Euro-N-CAP has been stricter. In order to correspond to the regulation, addition of a stiffened member or the like is required, and thus, an increase in body weight is unavoidable. The increase in body weight results in a decrease in fuel efficiency. Accordingly, the increase in a utilization of an ultra-high strength material, in which a thickness is capable of being thinned while a strength of a part is maintained, has been more preferable. On the other hand, in order to achieve the weight saving of the part as much as possible, a shape of the part becomes complicated. Thereby, further improvement of forming workability is required in the steel sheet. Particularly, in most cases, a high strength thin steel sheet is applied to a portion, in which bending deformation is mainly performed, such as a side sill. Accordingly, it is important to estimate a hole expansibility which is an index indicating a bendability or local ductility as formability of the high strength thin steel sheet. Moreover, since corrosion resistance is also required in the member, a hot-dip galvanizing or a galvannealing is applied to the high strength steel sheet used.
However, in general, if the strength of the steel sheet is increased, it is known that the forming workability such as the bendability or the hole expansibility is deteriorated.
For example, in the related art, steel sheets having improved the hole expansibility are suggested in Patent Documents 1 to 3.
Since the high strength steel sheet has much content of an alloying element and the alloying element is concentrated in a center portion of a sheet thickness, the hole expansibility is deteriorated. However, there is no related art which discloses a hardness difference between a surface portion of the steel sheet and a center portion of the steel sheet. Moreover, since a MnS having a large size becomes a fracture origin at the time of molding, it is assumed that a precipitation state of the MnS influences formability. However, there is no related art which discloses the precipitation state of the MnS.